What is Responsinator?

It's for website makers

Responsinator helps website makers quickly get an indication of how their responsive site will look on
the most popular devices. It does not precisely replicate how it will look, for accurate testing
always test on the real devices.

We recommend using Chrome on OSX 10.7+ with scroll bars set to "when scrolling" (System Preferences › General
› Show scroll bars › "when scrolling"). This means the device widths aren’t affected by scroll bars.